The present invention relates generally to signal source isolation stages. More particularly, this invention relates to solid state radio frequency isolation switches for mobile radio transceivers.
There are many applications where it is necessary to provide isolation for a signal source before coupling to a load having a given terminating impedance. One such application is in a mobile radio transceiver where minimal load-pull for an included variable-frequency programmable synthesizer is desired when selectively keying or de-keying a radio frequency (RF) transmittor thereof. When the mobile radio is keyed up for transmit, the voltage controlled oscillator (VCO) portion of the signal source is switched to the transmit frequency and the RF power amplifier (PA) stages are activated, or biased on. As the PA stages are biased on, the input impedance changes rapidly and "pulls" the VCO off frequency. This effect is commonly referred to as "load-pull".
Several techniques for providing isolation between a signal source and a variable impedance load in order to minimize load-pull are known. One commonly used arrangement imploys one or more high performance buffer stages to reduce the effects of the variable input impedance presented by the input to the RF PA as it is keyed up. Although this arrangement does reduce the effects of the variable load being presented to the signal source, the arrangement suffers from additional current drain and sets a limit as to how fast the mobile radio is able to transmit when given a command to do so. Such a limitation is of great consequence when designing mobile radios to operate in a high speed manner, such as for high speed data systems or for time-division-multiplex communication systems.
Several techniques for providing a load isolation function are known. One such arrangement imploys one or more buffer stages which help reduce the variation in load impedance presented to the signal source in order to maintain terminating impedance at a desired value. Although this arrangement is able to reduce the amount of variation in the load impedance presented to the signal source, it suffers from the disadvantage of trading off additional current consumption for each additional buffer stage added to improve the isolation further.
Solid state switches provide an attractive alternative to the foregoing buffer stage arrangement. Many such solid state switch arrangements are known, especially with regard to PIN diode transmit-receive switching networks. One known arrangement utilizing a PIN diode switching network in conjunction with several high performance buffer stages is the UHF Syntor-X radio available from Motorola Inc. However, in that arrangement, the circuit timing is such that the PIN diode switch is turned on immediately before transmitting and the PA is not biased on until the VCO is locked onto the transit frequency, leaving the load pull problem to be dealt with through the use of high performance buffer stages.
The present invention solves the above-mentioned problems by utilizing different timing of the PIN diode switch in conjunction with the key-up of the RF power amplifier without the use of buffer stages for isolation.
This invention represents a significant advance over the prior art by providing an isolation switch arrangement in which the effect of load-pull is practically eliminated for the VCO within the frequency synthesizer signal source.